New insights into the thermostability of bacterial ferredoxins: high-resolution crystal structure of the seven-iron ferredoxin from Thermus thermophilus

The crystal structure of the seven-iron ferredoxin from Thermus thermophilu s (FdTt) has been determined at 1.64 Angstrom resolution, allowing us to un veil the common mechanisms of thermostabilization within "bacterial-type" f erredoxins. FdTt and other homologous thermophilic seven-iron ferredoxins a re smaller than their mesophilic counterparts. Thermostabilizing features a re optimized in a minimal structural and functional unit, with an extensive cross-linking of secondary structure elements mediated by improved polar a nd hydrophobic interactions. Most of the potentially stabilizing features a re focused on the vicinity of the functional [3Fe-4S] cluster. The structur al [4Fe-4S] cluster is shielded in thermophilic FdTt by an increased number of polar interactions involving the two N-terminal residues. Comparisons w ith the hyperthermostable ferredoxin from Thermotoga maritima reveal that ( 1) a reduction in the number of non-glycine residues in strained conformati ons, (2) improved polar interactions within the common iron-sulfur cluster binding (beta alpha beta)(2) motif, and (3) an optimized charge distributio n at the protein surface, constitute a common strategy for increasing the t hermal stability of these ferredoxins.